Kinetics and mechanism of the reversible ring-opening of thiamine and related thiazolium ions in aqueous solution

Abstract

Kinetic studies of the ring-opening and reclosure reactions of thiamine and three other thiazolium ions (Q⁺) in aqueous solution, in the pH range 0–13, have been carried out by stopped-flow and conventional UV–VIS spectrophotometry. At high pH, ring-opening of thiamine exhibits a temporary diversion to the well-known ‘yellow form’. Otherwise, the ring-opening reactions are simply first-order in [OH^–], consistent with rate-limiting attack of hydroxide ion at C(2) of the Q⁺ ring, producing a pseudobase, T°, which rapidly consumes a second equivalent of hydroxide ion to form the ring-opened enethiolate, ETh^–. In contrast, ring closure of the enethiol in acidic solution exhibits rather complex kinetic behaviour; two processes are observed for most enethiols, including that derived from thiamine. Both the fast process (a) and the slower process (b) produce the thiazolium ion Q⁺ and they exhibit pH- and buffer-independent rate plateaux at low pH. Rapid, repetitive UV spectral scans and NMR spectral studies show that the two processes arise from the independent formation of Q⁺ from the two amide rotamers of the enethiol which do not equilibrate under the reaction conditions. The major amide rotamer (∼75%) gives rise to the fast process (a) and the minor rotamer to the slow reaction (b). The pH–rate profile and buffer catalysis studies reveal that the reclosure reaction undergoes a change in rate-limiting step from uncatalysed formation of T° at low pH to its general acid catalysed breakdown at higher pH. The latter process is characterized by a Brønsted α value of 0.70. Additionally, for process (b), a general base catalysed pathway for formation of T° can be observed, for which the Brønsted β value is 0.74. The mechanistic details of the ring-opening and reclosure pathways are discussed.